Side-emitting rod for use with an LED-based light engine

ABSTRACT

According to this invention, a side-emitting illumination device for uniformly distributing light is composed of an LED light source, a light-transmitting rod which permits total internal reflection, and outcoupling material affixed to an outer surface of the rod. Light enters the rod at one end and travels along the rod by total internal reflection. Light that hits the outcoupling material is angularly distributed based on the width of the outcoupling material.

BACKGROUND OF THE INVENTION

The invention relates to the distribution of light generated by one ormore LEDs through a rod. More particularly, the invention relates to awhite light emitting rod including paint distributed in such a way as toboth control the angular distribution of light leaving the rod, and toensure uniform light distribution along the length of the rod.

Recent side emitting rods, such as in U.S. Pat. No. 4,733,332, issued toAgency of Industrial Science and Technology and Daikin Industries, Ltd.and herein incorporated by reference, have used optical facets or finepowder down one side of the rod. These facets or powders serve tooutcouple light from the rod in a direction opposite to the facets, andwith a relatively narrow angular distribution. These rods arepotentially expensive to make, and do not perform well for white lightgenerated by mixing of multicolored LEDs, for example, red, green, andblue (RGB) LEDs.

A flexible rod material is used as an alternative outcoupling approach.This approach uses a narrow white strip embedded in a plastic material.Although this Lambertian scatter enhances the color of mixing of RGBwhite light, a uniform white stripe leads to a significant reduction inlight intensity from one end of the rod to the other.

It would be desirable to uniformly distribute high quality LED-generatedwhite light along the length of a rod, with a controllable angulardistribution of light outside the rod. Additionally, it would bedesirable that the efficiency with which light in the rod is coupledinto the desired distribution is high (preferably at least 70%). Itwould also be desirable to produce these rods cost effectively.

SUMMARY OF THE INVENTION

According to the invention, a side-emitting illumination device foruniformly distributing light is composed of a light source, alight-transmitting rod which permits total internal reflection, andoutcoupling material affixed to an outer surface of the rod. The lightsource, composed of one or more LEDs, passes light through the rod. Aslight is reflected off the inner surfaces of the rod, it impinges theoutcoupling material which controls the angular distribution of lightleaving the rod. This configuration ensures uniform light distributionalong the length of the rod.

In various embodiments of the invention, the rod can be a flexible rodor a rigid rod. The rod can be shaped like an ellipse, a square, or havea combination of straight and curved edges in cross-section. Thecombination of straight and curved edges can vary in configuration alongthe length of the rod.

In other embodiments, the outcoupling material is white paint or finewhite dots with varying packing density, distributed in such a way as tocontrol the angular distribution of light leaving the rod. The whitepaint or fine white dots are distributed in such a way as to ensureuniform light distribution along the length of the rod.

In another embodiment the side-emitting illumination device includes amirror at an end of the rod away from the light source to reflect lightwhich reaches the end of the rod, increasing the outcoupling efficiencyof the system.

The invention provides several advantages. The side-emittingillumination device allows for high quality white light distributionalong the length of a rod, with a controllable angular distribution oflight outside the rod. Additionally, the efficiency with which light inthe rod is coupled into the desired distribution is high. Further, theheat generated by the light source is separate from the rod, and LEDwhite light generated from red green and blue LEDs allows for changingcolor temperatures and adding dynamic color effects. Also, the size ofthe light engine system is smaller than currently required sources.Finally, the side-emitting illumination device can be produced costeffectively.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 side-emitting illumination device according to the invention;

FIG. 2A is a graph showing the angular distribution of light determinedby the width of the paint stripe of 25°;

FIG. 2B is a graph showing the angular distribution of light determinedby the width of the paint stripe of 45°;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, side-emitting illumination device 12 incorporates aRGB LED light source 5 which generates white light that enters rod 6 atone end. The white light travels through the rod 6 by total internalreflection (TIR). Rod 6 has a stripe of white paint 7 along its length.Light which hits the white paint is scattered and may no longer meet theTIR condition, which exists in the rod 6, and a portion of the lightexits the side of rod 6.

In FIG. 1 a mirror 8 is placed at the end of rod 6 opposite the LEDlight source 5. This allows light that reaches the end of the rod 6 tobe reflected and pass back down the length of the rod 6, increasing theoutcoupling efficiency of the system. The outcoupling efficiency doesdepend, in part, on the input angular light distribution. Lighttraveling at a high angle to the optical axis of the rod is outcoupledmore rapidly than light that is nearly parallel to the optical axis ofthe rod 6. The angular distribution of input light should be optimizedfor the system. Generally, it is better to have a distribution with lowflux along the optical axis of the rod. This can be controlled by thearrangement of LEDs and optics inside the LED light engine used as asource 5 for the rod 6.

The angular distribution of light (in the direction around the rod) isdetermined by the width of the paint stripe. Two examples of thedistribution are shown in FIGS. 2A and 2B. Referring to FIG. 2A, thepaint width is 25° (paint width is henceforth quoted as an angularwidth). Referring to FIG. 2B, the paint width is 45°. Broader outputdistribution results from increased angular width of the paint.

The calculations of angular width for FIGS. 2A and 2B are based on around cross section rod. It is also possible to use othercross-sections. In particular, the cross-sections may be elliptical,square, or more generally, any combination of straight and curved edges,possibly varying in configuration along the length of the rod. Thesegeneralizations allow further control of the output angulardistribution, permitting an inexpensive customization of distinctillumination products. It is also possible to vary the width of thepaint stripe along the rod to gain still more control of the outputangular distribution.

In many applications it is desirable to have the outcoupled lightintensity uniform along the rod. This can be achieved by correctlyvarying the effective paint density along the rod 6. The paint densityneeds to be lower close to the light source than far away from the lightsource. This is easily achieved by replacing the solid stripe of paintwith a series of stripes perpendicular to the length of rod 6, andvarying the spacing between these small stripes and/or the width ofthese stripes (in the direction along the rod). Alternatively, the paintstripe can include fine white dots with varying packing density.

A potential application for the above-mentioned embodiments includesreplacing lamps found in commercial supermarket freezers. The LED systemprovides a number of advantages recited above.

The preceding expressions and examples are exemplary and are notintended to limit the scope of the claims which follow.

1-18. (cancelled)
 19. A side-emitting illumination device for uniformlydistributing light comprising: a light source, a light-transmitting rodwhich permits substantially total internal reflection, and outcouplingmaterial affixed to an outer surface of the rod, wherein the angularwidth of the outcoupling material affixed to an outer surface of the rodcontrols the angular distribution of light leaving the side of the rod.20. The side-emitting illumination device of claim 19, wherein the lightsource further comprises a plurality of LEDs.
 21. The side-emittingillumination device of claim 20, wherein the plurality of LEDs includesat least a red, a green, and a blue LED which, when mixed, generatewhite light.
 22. The side-emitting illumination device of claim 21,wherein the array of red, green, and blue LEDs can be mixed to generatewhite light chromaticity.
 23. The side-emitting illumination device ofclaim 21, wherein the array of red, green, and blue LEDs can be mixed togenerate dynamic color effects.
 24. The side-emitting illuminationdevice of claim 19, wherein the rod is a flexible rod.
 25. Theside-emitting illumination device of claim 19, wherein the rod is arigid rod.
 26. The side-emitting illumination device of claim 19,wherein the outcoupling material is paint.
 27. The side-emittingillumination device of claim 26, wherein the paint is white paint. 28.The side-emitting illumination device of claim 27, wherein the whitepaint is distributed in such a way as to control the angulardistribution of light leaving the rod.
 29. The side-emittingillumination device of claim 27, wherein the white paint is distributedin such a way as to ensure uniform light distribution along the lengthof the rod.
 30. The side-emitting illumination device of claim 19,wherein the rod is an elliptical rod in cross-section.
 31. Theside-emitting illumination device of claim 19, wherein the rod is asquare rod in cross-section.
 32. The side-emitting illumination deviceof claim 19, wherein the rod is a combination of straight and curvededges in cross-section.
 33. The side-emitting illumination device ofclaim 32, wherein the combination of straight and curved edges vary inconfiguration along the length of the rod.
 34. The side-emittingillumination device of claim 19, wherein the outcoupling materialcomprises a combination of white paint and fine dots with varyingpacking density.
 35. The side-emitting illumination device of claim 19,wherein the device further comprises a mirror at an end of the rod awayfrom the light source.
 36. The side-emitting illumination device ofclaim 35, wherein the mirror reflects light that travels the entirelength of the rod.
 37. A method of controlling the angular distributionof light leaving the side of a side-emitting illumination device foruniformly distributing light comprising: providing a light-transmittingrod which permits substantially total internal reflection with anoutcoupling material along its side; controlling the width of theoutcoupling material to achieve a desired angular distribution of lightleaving the side of the rod; and illuminating the light-transmitting rodwith a light source.
 38. A side-emitting illumination device fordistributing light comprising: a light source, a light-transmitting rodwhich permits substantially total internal reflection, and outcouplingmaterial affixed to an outer surface of the rod, wherein the outcouplingmaterial exclusively controls the angular distribution of light leavingthe side of the rod.